Modern stepper lens trains are typically designed for the transmission of an image of electronic features to a substrate being processed. Unfortunately, specifications for such images change--the most common change being that the image requires increasingly smaller features fabricated to higher tolerances.
As the specifications change--usually by becoming smaller and more exacting--the requirements of imaging frequently demand upgrading of the stepper optics. Unfortunately, such upgrading frequently necessitates replacement of the stepper--or at least the stepper optics.
Additionally, building a stepper optical train can require exacting placement of conventional classical optics. Accordingly, it would be desirable if error of an optical train built to relatively low tolerance could have the errors of such optical trains mapped and then up-graded to a higher tolerance. Or, alternatively, state-of-the-art imaging objective optics can be improved to perform beyond their intended specifications.
In Plate Correction of Imaging Systems, now U.S. Pat. No. 5,392,119 issued Feb. 21, 1997, stepper correction utilizing two plates is disclosed. In this patent, two planes are taken through the stepper optical train. Taking an "ideal" reticle broadcasting its image to an "ideal" image to be produced by the stepper optical train, first mapping the ideal image to a corrected reticle image occurs which displaces the corresponding reticle points in accordance with the field curvature and distortion of the mapped stepper system. Thereafter, the first plate is given sufficient displacement to deviate light from the displaced reticle points to coincidence to a perfect ray trace path at the second corrector plate. At the second corrector plate, the light rays will be in coincidence to--but not necessarily have proper direction to--their corresponding points on the ideal image. Consequently, at the second corrector plate, deviation of the light occurs to deflect the direction of the light into coincidence to a "perfect" ray trace path to an ideal image. Correction of the optical stepper results. This Patent is incorporated by reference to this application as if set forth fully herein.
In Plate Correction Technique for Imaging Systems, U.S. patent application Ser. No., 08/592,703 filed Jan. 26, 1996, again two corrector plates are utilized. In this system a stepper configuration is modified by corrector plates so that an image from a reticle plane is projected to an ideal image at an object plane. The preferred location of plates correcting for curvature of field is at the reticle plane, it being noted that distortion introduced by a field corrector plate at this location is generally negligible. Three generalized cases for correction are therefore discussed. The first case is field curvature correction where the correcting plate is registered to or close to the reticle plate; it is shown that induced distortion correction may generally be ignored. The second case is where field curvature correction cannot be placed next to the reticle plate, but the field curvature induced by required distortion correction is negligible. Finally, the case is considered where field curvature induced by the correction of distortion is not negligible and must be compensated for in overall correction. This disclosure is a technique for 2-plate correction of imaging systems with greatly increased ability to correct for curvature of field. General formulas can be derived for n-plate correction using this same technique. This Application is incorporated hereto by reference as if fully set forth herein.
In the present disclosure, stepper correction has been reduced to a single plate.